Kingdom of Ife

Figure of a king, Ita Yemoo, a copper alloy sculpture dating from the late 13th to the early 15th centuries. Photograph: Karin L Wills/Museum for African Art/National Commission for Museums and Monuments, Nigeria Karin L Wills/Museum for African Art/National Commission for Museums and Monuments, Nigeria. Kingdom of Ife, British Museum/PR

This is an exceptional exhibition, even by the high standards the British Museum has established in recent years. It is extraordinary because it brings together such a large number of masterpieces that have rarely or never been exhibited outside Nigeria before – and when I say masterpieces, I mean artworks that rank with the Terracotta Army, the Parthenon or the mask of Tutankhamun as treasures of the human spirit.

For European artists a century ago, African sculpture was powerful precisely because it did not conform to the smooth idea of beauty that Picasso's generation had been brought up on – ideas that went back to classical Greece. But they had not seen the art of Ife, a medieval city state that flourished from the 12th to 15th centuries in West Africa, trading across the Sahara with the Islamic Mediterranean world.

The superb sculpted heads in this exhibition – statues of sick people, monuments to warriors, royal heads whose strange vertical scars tell of the ceremonies of the court – were first rediscovered in quantity in an amazing find on a building site in the modern Nigerian city of Ife in 1938. This art was so different and unexpected, so "un-African", that one of its first students thought it must be the lost art of Atlantis.

But these works were not Greek, let alone from Atlantis. The faces that gaze coolly past you from these cases are challenging and formidable in their beauty. And they are disturbing to anyone who has any lingering belief in the uniqueness of European art. Sculptors in Ife imitated the human face as accurately and sensitively as any Greek, and matched the Greek feeling for harmony, balance and proportion.

What we see here is an African classical art – by which I mean an art with a strong concept of order that gives it a special authority, whether it comes from Athens, China or Ife. Like that of ancient Egypt, the art of Ife is perfect, remote, godlike and yet – as with Egypt – when you look again it is highly observational, rooted in the real life of this lost civilisation.

Ife remains mysterious. The catalogue admits there's so much still to learn about this art and the world that created it. Hopefully this exhibition will be the starting point for new archaeology. It elicits awe. To behold these royal heads is to travel to a fabled realm far beyond your imagination, a place richer than Atlantis.